This invention relates generally to a system for administering general anesthetics in the gaseous state and more particularly to an anesthesia rebreathing system, comprised of a permanent (or reusable) structural portion and a disposable (or single use) structural portion.
U.S. Pat. Nos. 3,814,091 and 3,901,230 disclose an anesthesia rebreathing system comprised of a reusable structural portion and a single use structural portion cooperatively configured to minimize the possibility of crosscontamination between patients. The system is characterized by a geometry which preferentially vents expired alveolar gas, rich in CO.sub.2, while preserving initially expired dead space gas to thus minimize the need for CO.sub.2 absorption.
In one embodiment disclosed in U.S. Pat. No. 3,814,091, the single use portion comprises a patient circuit generally referred to as a circle including both inspiratory and expiratory patient breathing tubes. In a second disclosed embodiment, the patient circuit comprises a single breathing tube alternately used for inspiration and expiration. Each circuit embodiment also incorporates an overflow tube whose entrance is located very close to the patient. The overflow tube exits at a reusable patient overflow (commonly referred to as "pop-off") valve which is located close to an anesthesia machine where it can be conveniently controlled by an attending anesthetist. By locating the overflow tube entrance close to the patient, it functions to preferentially vent alveolar gas through the overflow valve and save dead space and unbreathed gas within the tubing and reservoir of the patient circuit. The patient reservoir is disclosed as comprising one chamber of a dual chamber bag. The overflow valve is disclosed as being operable in two different modes, i.e. (1) as a manually controlled variable orifice for spontaneous ventilation and (2) as an automatically controlled valve responding to a positive control pressure for manually assisted or mechanically controlled ventilation.
U.S. Pat. No. 3,901,230 discloses an enhanced system characterized by the inclusion of an improved subsystem for controlling the gas volume and pressure in the patient circuit. The subsystem (which has since become known as a "ventilator/isolator" circuit) includes a rigid container (forming part of the system's reusable portion) within which a single chamber patient breathing bag (forming part of the single use portion) is accommodated. The pressure within the rigid container is controlled (1) during manually assisted or controlled ventilation, by an attending anesthetist squeezing an outside bag and (2) during mechanically controlled ventilation by a conventional mechanical ventilator. The pressure variations in the rigid container are applied to the patient circuit via the flexible walls of the patient breathing bag. Crosscontamination is eliminated in such a system because the patient expired gas cannot come into contact with reusable portion components exposed to inspired gas.
More specifically, the enhanced system of U.S. Pat. No. 3,901,230 can be functionally viewed as including a patient circuit and a ventilator/isolator circuit for controlling gas volume and pressure in the patient circuit. The system can be structurally viewed as including a single use portion and a reusable portion. The system is configured so that the single use portion forms most of the patient circuit with the reusable portion forming the ventilator/isolator circuit and part of the patient circuit, e.g. the adjustable patient overflow valve.
The inventions disclosed in said U.S. Pat. Nos. 3,814,091 and 3,901,230 have been embodied in an anesthesia system marketed commercially since about 1976 as the ANTROL system by Diamed, a division of Illinois Tool Works, Inc. The ANTROL system with ventilator/isolator includes the following independently operable user controls:
1. Patient pop-off valve, variably adjustable between open and closed positions and having a ventilator position at which the pop-off valve is controlled by the pressure in the rigid container;
2. Ventilator/isolator selector valve, movable between first and second positions for respectively coupling either the outside bag or mechanical ventilator to the rigid container;
3. Ventilator/isolator fill valve for selectively filling the ventilator/isolator circuit; e.g. the ventilator bellows or the outside bag; and
4. Ventilator/isolator dump valve for selectively relieving pressure from the ventilator/isolator circuit; e.g. the ventilator bellows or the outside bag.
The foregoing user controls are used by the attending anesthetist in conjunction with anesthesia machine controls (e.g. flush valve) to maintain proper gas volume and pressure in the patient and ventilator/isolator circuits.